1. Field of the Invention
This invention relates to a stepping motor driver circuit, and more particularly to a stepping motor driver circuit with a power saving function for use with a disk drive unit of a floppy disk apparatus of an information processing system or a like apparatus.
2. Description of the Prior Art
When a stepping motor operates in a stepping operation, high torque is required, and consequently, high driving current is required. However, when the stepping motor is in holding mode, high torque is not required, and accordingly, the driving current can be reduced. Therefore, it is a common practice to provide a stepping motor driver circuit with a power saving function so that excessive current will not be supplied to the stepping motor when the stepping motor is in holding mode.
A conventional stepping motor driver circuit of the type mentioned is constituted from, as shown in FIG. 1, a bridge circuit 3 for a driving coil 4 of a stepping motor, inverters I1 to I3 constituting a driving circuit for the driving bridge circuit 3, and a pair of holding circuits 5 and 6 for setting the holding current at a lower level than the operating current of the stepping motor when the stepping motor is in holding mode.
The bridge circuit 3 is constituted from one push-pull circuit 31 constituted from a P-channel MOS transistor P31 and an N-channel MOS transistor N31 connected in series to each other between a power source VD and a ground VS and having output O1 at the mid-point between them, and another push-pull circuit 32 constituted from a P-channel MOS transistor P32 and an N-channel MOS transistor N32 connected in series to each other between the power source VD and the ground VS and having an output O2 at the midpoint between them, similar to the other push-pull circuit 31.
Inverters I1 and I2, which have the same polarity as input IN, drive push-pull circuit 31, and inverter I3, which has the opposite polarity to input IN, drives push-pull circuit 32.
Holding circuit 5 is constituted from an operational amplifier A51, a reference voltage source VR51 of voltage vR, and a switch circuit S51. Similarly, holding circuit 6 is constituted from an operational amplifier A61, a reference voltage source VR61 of voltage vR, and a switch circuit S61.
Switch circuit S51 selects contact b when the stepping motor is in ordinary operating mode, in which high torque is required, or when input IN is at an "H" (high) level, but it selects the other contact a when the stepping motor is in holding mode and input IN is at an "L" (low) level. On the other hand, switch circuit S61 selects contact b when the stepping motor is in ordinary operating mode, in which high torque is required, or when input IN is at an "L" level, but it selects the other contact a when the stepping motor is in holding mode and input IN is at an "H" level.
Since the stepping motor actually has two coils, it is provided with two similar driving circuits which drive it stepwise in synchronism with each other.
Operation of the conventional stepping motor driver circuit is described below.
First, ordinary operation is described. In this instance, switch circuits S51 and S61 both select contacts b. Accordingly, the gates of transistors P31 and N31 and the gates of transistors P32 and N32 of push-pull circuits 31 and 32 are connected in common to each other. When input IN is at an "H" level, the output of inverter I2 presents an "H" level while the output of inverter I3 presents an "L" level. Consequently, transistors N31 and P32 are ON, and transistors N32 and P31 are OFF. Accordingly, the output O2 of push-pull circuit 32 applied to coil 4 is voltage vD of power source VD, and the output O1 of push-pull circuit 31 is voltage vS of ground VS. As a result, the load current iD=vD/R, which depends upon the direct current resistance R of coil 4, flows from the output O2 side to the output O1 side through coil 4. In this instance, since the on-resistances of transistors N1 and P32 are low, they are ignored and regarded as equal to 0.
Next, when input IN changes to an "L" level, the output of inverter I2 is changed to an "L" level while the output of inverter I3 is changed to an "H" level, and consequently, transistors N31 and P32 are turned OFF and transistors N32 and P31 are turned ON. Accordingly, output O1 of push-pull circuit 31 applied to coil 4 is voltage Vd of power source VD, and output O2 of push-pull circuit 32 is voltage vS of ground VS. As a result, conversely to the case described above, load current iD flows from the output O1 side to the output O2 side through coil 4.
Operation of the stepping motor in holding mode (power saving mode) will next be described. The negative input of operational amplifier A51 is connected to reference power source VR51 while the positive input is connected to the output O1 of push-pull circuit 31, and the output of operational amplifier A51 is connected to the gate of transistor P31 of push-pull circuit 31 via contact a of switch circuit S51. Similarly, the negative input of operational amplifier A61 is connected to reference power source VR61 while the positive input is connected to the output O2 of push-pull circuit 32, and the output of operational amplifier A61 is connected to the gate of transistor P32 of push-pull circuit 32 via contact a of switch circuit S61.
First, when the stepping motor is in holding mode and input IN is at an "L" level, switch circuit S51 selects contact a and switch circuit S61 selects contact b. Further, since the output of inverter I2 presents an "L" level while the output of inverter I3 presents an "H" level, transistors N31 and P32 are OFF and transistors P31 and N32 are ON. Accordingly, the output O2 of push-pull circuit 32 is voltage vS of ground vS. Negative feedback is applied to transistor P31 by operational amplifier A51, and accordingly, the output O1 of push-pull circuit 31 presents the equal potential to reference voltage vR. Accordingly, the voltage applied across coil 4 of the stepping motor is reference voltage vR. As a result, current iH flowing through coil 4 is iH=vR/R. Accordingly, power consumption can be reduced by vD.times.(vD/R -vR/R) as compared with power consumption when the stepping motor is in operating mode.
FIG. 2 is a circuit diagram showing an example of a configuration of operational amplifiers A51 and A52. As shown in FIG. 2, each of operational amplifiers A51 and A52 is a voltage feedback amplifier constituted from a differential amplifier, a pair of output buffer amplifiers for a differential pair of the differential amplifier, a constant-current source for the differential amplifier and the buffer amplifiers, and some other elements, and employs a total of eleven transistors N51 to N56 and P51 to P55.
The conventional stepping motor driver circuit described above is disadvantageous in that, since it employs an operational amplifier of the voltage feedback type which includes a comparatively large number of component elements such as transistors, it is large in pellet size and high in cost when it is constituted into a semiconductor integrated circuit.